1. Field of the Invention
The present invention relates to a cathode member for an electronic tube and more particularly, to a method of fabricating a cathode member of an electronic tube such as cathode-ray tubes (CRTs), traveling-wave tubes, and so on. in which the thermal activation process is completed in a shortened time and the maximum cathode current is effectively prevented from lowering, and an electronic tube equipped with the cathode member.
2. Description of the Prior Art
A conventional method of fabricating a cathode member or pellet using a hot isostatic pressing (HIP) process is disclosed in the Japanese Non-Examined Patent Publication No. 8-50849 published in 1996, which corresponds to the U.S. Pat. No. 5,757,115 issued in May 1998.
In this conventional method, first, a nickel (Ni) alloy powder containing magnesium (Mg) and silicon (Si) as reducing agents, a barium carbonate (BaCO.sub.3) powder, strontium carbonate (SrCO.sub.3) powder, and a calcium carbonate (CaCO.sub.3) powder are well mixed together using a ball mill, thereby generating a powder mixture of these powders.
Second, the powder mixture is filled into a rubber molding die and then, the molding die is sealed. The powder mixture filled in the sealed molding die is subjected to a cold isostatic pressing (CIP) process, thereby forming a molded material.
Third, the molded material is introduced into a glass capsule, and then the capsule is sealed and held in vacuum. The capsule is then subjected to a HIP process. thereby sintering the molded material.
Fourth, the sintered, molded material is taken out of the capsule and then, it is subjected to machining processes such as cutting and polishing. Thus, a cathode pellet with a specific geometry is produced.
Finally, the cathode pellet is inserted into a cathode cap. The cathode cap with the inserted cathode pellet is inserted into the inside of a cathode sleeve and is fixed thereto by welding. Thus, a cathode assembly is fabricated.
The conventional cathode assembly including the cathode pellet, the cathode cap, and the cathode sleeve described above is mounted on a CRT in the following way.
First, the cathode assembly is fixed onto an electron gun together with a heater. Next, the electron gun with the cathode assembly and the heater is fixed to a glass valve of a CRT.
Subsequently, an electric current is supplied to the heater of the cathode assembly for the purpose of heating the cathode pellet while the inside of the glass valve is evacuated. Thus, ternary co-precipitated carbonate of Ba, Sr, and Ca, i.e., (Ba,Sr,Ca)CO.sub.3, contained in the cathode pellet is chemically decomposed and changed to ternary oxide of Ba, Sr, and Ca, i.e., (Ba,Sr,Ca)O.
After the glass valve of the CRT is sealed, the heater is supplied with an electric current again to thereby heat the cathode pellet. Thus, the electron emission capability of the cathode member is increased or activated.
The activation process of increasing the electron emission capability of the cathode pellet by simply heating is termed the "thermal activation". On the other hand, the activation process of increasing the electron emission capability of the cathode pellet by applying respectively positive and negative electric potentials to the cathode and anode of the electron gun so that the activation is conducted while inducing the electron emission therefrom is termed the "current activation".
The electron emission capability of the cathode pellet is raised up to a practically high level through these two activation processes, i.e., "thermal and current activations" processes.
The cathode pellet fabricated by the above-described conventional method has the following three problems.
First, the necessary time and temperature for the thermal activation process are long and high, respectively, and consequently, the following disadvantages (i) to (iii) are caused.
(i) Since the cathode cap and the cathode sleeve are made of Nichlome alloy whose heat-resistance property is comparatively low, the cathode cap and sleeve tend to be thermally deformed in the thermal activation process. PA1 (ii) A lot of barium (Ba) contained in the cathode pellet tends to evaporate due to high temperature in the thermal activation process, thereby shortening the lifetime of the cathode. PA1 (iii) The evaporated barium (Ba) tends to be deposited onto a grid located near the cathode, which cases electron emission not only from the cathode pellet but also from the grid. The electron emission from the grid will generate unnecessary or undesired illumination on the screen of the CRT. PA1 (a) A nickel powder and a rare-earth-metal oxide powder are provided. PA1 (b) The nickel powder and the rare-earth-metal oxide powder are uniformly mixed together, thereby producing a first powder mixture. PA1 (c) The first powder mixture is heated in a hydrogen atmosphere, an inert atmosphere, or a vacuum atmosphere, thereby producing an intermetallic compound of nickel and the rare-earth metal in the first powder mixture. PA1 (d) The first powder mixture containing the intermetallic compound is uniformly mixed with an electron-emissive agent powder, thereby producing a second powder mixture. PA1 (e) The second powder mixture is sintered by a HIP process, thereby forming a cathode member.
Second, the increase of the electron emission capability of the cathode pellet due to the current activation process is not sufficient.
Specifically, the electron emission capability is usually increased by the current activation process at a much lower temperature than the thermal activation process. Therefore, to avoid the above disadvantages (i) and (ii) about the thermal deformation of the cathode cap and sleeve and the Ba evaporation, it is typical that the time for the thermal activation process is set as short as possible and at the same time, the current activation process is chiefly used for increasing the electron emission capability.
In the cathode pellet fabricated by the above-described conventional method, however, the electron emission capability is scarcely increased by the current activation process. Thus, the necessary time for the thermal activation process is unable to be shortened, which makes the time for the current activation process comparatively long. Consequently, the above-described disadvantages (i) to (iii) will occur.
Third, the maximum cathode current (MIk) is unsatisfactorily prevented from lowering during practical operation.
Specifically, in the cathode pellet fabricated by the above-described conventional method, the lowering of the maximum cathode current (MIk) during practical operation is extremely small compared with the conventional oxide-coated cathode pellet, resulting in a long lifetime. However, the maximum cathode current (MIk) tends to gradually decrease during the practical operation even if free Ba remains in the cathode pellet.
An ideal cathode pellet is a pellet whose maximum cathode current (MIk) is never lowered while free Ba remains in the cathode pellet. It is not said that the above-described conventional cathode pellet is the ideal cathode pellet.